This application discloses an invention that is related, generally and in various embodiments, to a system and method for limiting AC inrush current when energizing a power supply.
For many power supplies, when power is suddenly applied to the input transformer of the power supply, the transformer is generally subjected to a substantial surge of current therethrough. This inrush current comprises a first component that is due to the charging of DC capacitors in the power supply, and a second component that is due to the saturation of the iron core of the transformer.
In general, secondary current only flows during the first cycle of inrush current, and the DC capacitors are fully charged at the end of the first cycle. The inrush current produces an overshoot of the DC voltage. Due to size and cost constraints, the number of DC capacitors that may be utilized to lessen the overshoot of DC voltage during inrush is limited. The overshoot in the DC capacitor voltage during inrush is due to energy stored in stray or deliberate inductance(s) at the large charging currents present during inrush. In general, the charging currents last for less than one cycle. The same capacitor charging currents are induced into the primary winding, reduced by the turns-ratio of the transformer.
In addition to the charging currents induced onto the primary winding from the secondary winding(s), there can be additional currents in the primary winding only due to saturation of the iron core. These saturation currents repeat over several subsequent cycles, even though the secondary charging currents are essentially zero after the first cycle. These multiple pulses of primary current stress the primary windings of the transformer, as well as the power distribution system. While the iron core is saturated, the inrush current is limited mainly by the self-inductance of the primary winding, so the inrush current can be very large. The peak primary currents are progressively diminished on each successive cycle, as the DC offset flux from the transient slowly decays, so that saturation occurs later in each cycle. However, the pulses of saturation current can last 20 cycles or more.